Gentica Molecular em Anlises Clnicas

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Genética Molecular em Análises Clínicas

• MHC

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Genetic basis of transplant rejection
Transplantation of skin between strains showed
that rejection or acceptance was dependent
upon the genetics of each strain
ACCEPTED
REJECTED
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Immunological basis of graft rejection
Transplant rejection is due to an
antigen-specific immune response with
immunological memory.
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Immunogenetics of graft rejection
ACCEPTED
REJECTED
Mice of strain (A x B) are immunologically
tolerant to A or B skin
Skin from (A x B) mice carry antigens that are
recognised as foreign by parental strains
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Major Histocompatibility Complex  MHC
In mice the MHC is called H-2 Rapid graft
rejection segregated with a cell surface antigen,
Antigen-2 Inbred mice identical at H-2 did not
reject skin grafts from each other MHC genetics
in mice is simplified by inbred strains
In humans the MHC is called the Human Leukocyte
Antigen system HLA Only monozygous twins are
identical at the HLA locus The human population
is extensively outbred MHC genetics in humans is
extremely complex
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MHC molecules
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Differential distribution of MHC molecules
Tissue MHC class I MHC class II T
cells /- B cells
Macrophages Other
APC Epithelial cells of
thymus Neutrophils
- Hepatocytes - Kidney
- Brain
- Erythrocytes - -
Cell activation affects the level of MHC
expression The pattern of expression reflects the
function of MHC molecules Class I is involved in
anti-viral immune responses Class II involved in
activation of other cells of the immune system
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Overall structure of MHC class I molecules
?3 domain ?2m have structural amino acid
sequence homology with Ig C domains Ig GENE
SUPERFAMILY
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MHC class I molecule structure
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Structure of MHC class I molecules
?1 and ?2 domains form two segmented ?-helicies
on eight anti-parallel ?-strands to form an
antigen-binding cleft.
Properties of the inner faces of the helicies and
floor of the cleft determine which peptides bind
to the MHC molecule
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Overall structure of MHC class II molecules
No b-2 microglobulin
?2 ?2 domains have structural amino acid
sequence homology with Ig C domains Ig GENE
SUPERFAMILY
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MHC class II molecule structure
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MHC-binding peptides
Each human usually expresses 3 types of MHC
class I (A, B, C) and 3 types of MHC class II
(DR, DP,DQ)
The number of different T cell antigen receptors
is estimated to be 1,000,000,000,000,000 Each of
which may potentially recognise a different
peptide antigen
How can 6 invariant molecules have the capacity
to bind to 1,000,000,000,000,000 different
peptides?
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Anchor residues and T cell antigen
receptor contact residues
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MHC molecules can bind peptides of different
length
Complementary anchor residues pockets provide
the broad specificity of a particular type of MHC
molecule for peptides
Peptide sequence between anchors can vary Number
of amino acids between anchors can vary
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Peptide antigen binding to MHC class II molecules
Anchor residues are not localised at the N and
C termini Ends of the peptide are in extended
conformation and may be trimmed Motifs are
less clear than in class I-binding peptides
Pockets are more permissive
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How can 6 invariant molecules have the capacity
to bind to 1,000,000,000,000,000 different
peptides with high affinity?
MHC molecules Adopt a flexible floppy
conformation until a peptide binds Fold around
the peptide to increase stability of the
complex Use a small number of anchor residues
to tether the peptide this allows different
sequences between anchors and different lengths
of peptide
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MHC molecules are targets for immune evasion by
pathogens
T cells can only be activated by interaction
between the antigen receptor and peptide antigen
in an MHC molecule Without T cells there can
be no effective immune response There is
strong selective pressure on pathogens to evade
the immune response The MHC has evolved two
strategies to prevent evasion by
pathogens More than one type of MHC
molecule in each individual Extensive
differences in MHC molecules between individuals

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Example If MHC X was the only type of MHC
molecule
Population threatened with extinction
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Example If each individual could make two MHC
molecules, MHC X and Y
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Example If each individual could make two MHC
molecules, MHC X and Yand the pathogen mutates
.until it mutates to evade MHC Y
Population threatened with extinction
Survival of individual threatened
The number of types of MHC molecule can not be
increased ad infinitum
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Populations need to express variants of each type
of MHC molecule
The rate of replication by pathogenic
microorganisms is faster than human
reproduction In a given time a pathogen can
mutate genes more frequently than humans and can
easily evade changes in MHC molecules The
number of types of MHC molecules are limited
To counteract the flexibility of pathogens The
MHC has developed many variants of each type of
MHC molecule These variants may not
necessarily protect all individuals from every
pathogen, but will protect the population from
extinction
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Variant MHC molecules protect the population
From 2 MHC types and 2 variants. 10 different
genotypes
Variants of each type of MHC molecule increase
the resistance of the population from rapidly
mutating or newly encountered pathogens without
increasing the number of types of MHC molecule
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Molecular basis of MHC types and variants
POLYGENISM Several MHC class I and class II genes
encoding different types of MHC molecule with a
range of peptide-binding specificities.
POLYMORPHISM Variation gt1 at a single genetic
locus in a population of individuals MHC genes
are the most polymorphic known
The type and variant MHC molecules do not vary in
the lifetime of the individual The diversity in
MHC molecules exists at the population level This
sharply contrast diversity in T and B cell
antigen receptors which exists within the
individual
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Simplified map of the HLA region
Polygeny CLASS I 3 types HLA-A, HLA-B, HLA-C
(sometimes called class Ia genes)
CLASS II 3 types HLA-DP HLA-DQ HLA-DR.
Maximum of 9 types of antigen presenting
molecule allow interaction with a wide range of
peptides.
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Detailed map of the HLA region
http//www.anthonynolan.org.uk/HIG/data.html July
2000 update
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Map of the Human MHC from the Human Genome Project
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Other genes in the MHC
MHC Class 1b genes Encoding MHC class I-like
proteins that associate with ?-2
microglobulin HLA-G interacts CD94 (NK-cell
receptor). Inhibits NK cell attack of foetus/
tumours HLA-E binds conserved leader peptides
from HLA-A, B, C. Interacts with CD94 HLA-F
function unknown

MHC Class II genes Encoding several antigen
processing genes HLA-DM? and ?, proteasome
components (LMP-2 7), peptide
transporters (TAP-1 2), HLA-DO? and DO? Many
pseudogenes
MHC Class III genes Encoding complement proteins
C4A and C4B, C2 and FACTOR B TUMOUR NECROSIS
FACTORS ? AND ?
Immunologically irrelevant genes Genes encoding
21-hydroxylase, RNA Helicase, Caesin kinase Heat
shock protein 70, Sialidase
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Polymorphism in the MHC
Variation gt1 at a single genetic locus in a
population of individuals Each polymorphic
variant is called an allele In the human
population, over 1,200 MHC alleles have been
identified
492 alleles
657 alleles
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Allelic polymorphism is concentrated in the
peptide antigen binding site
Polymorphism in the MHC affects peptide antigen
binding Allelic variants may differ by 20 amino
acids
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Diversity of MHC molecules in the individual
HAPLOTYPE 1
HAPLOTYPE 2
MHC molecules are CODOMINANTLY expressed Two of
each of the six types of MHC molecule are
expressed
Genes in the MHC are tightly LINKED and usually
inherited in a group The combination of alleles
on a chromosome is an MHC HAPLOTYPE
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Inheritance of MHC haplotypes
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Errors in the inheritance of haplotypes generate
polymorphism in the MHC by gene conversion and
recombination
RECOMBINATION between haplotypes
In both mechanisms the type of MHC molecule
remains the same, but a new allelic variant may
be generated